The name ABS is derived from the initial letters of acrylonitrile, butadiene, and styrene, the most conventionally used monomers for the preparation of graft copolymers.
Thermoplastically deformable ABS polymers are heterogeneous plastics consisting of two phases. One phase is essentially obtained by polymerizing styrene and acrylonitrile in the presentce of an elastic substance derived from at least one homopolymer or copolymer of butadiene. This material is dispersed as an internal discontinuous phase in a thermoplastic styrene-acrylonitrile (SAN) copolymer as the matrix. In principle, saturated elastomeric components can also serve as the elastic substance for the discontinuous phase.
The physical properties of ABS polymers are only partially attributable to the properties of their individual monomeric and/or polymeric components. One of the most important and desirable properties, the impact strength, is determined by the two-phase structure formed by the rubber phase embedded in the hard, brittle thermoplastic matrix. The heat resistance of these ABS polymers is inadequate, however, for a number of applications, particularly for the motor industry and for household and electrical appliances.
The replacement of styrene by .alpha.-methylstyrene in ABS is known from U.S. Pat. No. 3,594,453.
Furthermore it is known from a number of publications that the partial or total replacement of styrene by .alpha.-methylstyrene in SAN has a beneficial effect on the heat resistance. For example, U.S. Pat. No. 3,010,936 discloses a polymer composition based on a copolymer of .alpha.-methylstyrene and acrylonitrile or a terpolymer of .alpha.methylstyrene, styrene and acrylonitrile mixed with a copolymer of styrene and acrylonitrile onto a butadiene rubber. The polymer composition disclosed therein achieves a sufficient degree of heat resistance by requiring a relatively large proportion of copolymer or terpolymer. The composition thereby suffers, however, because this requirement results in an unacceptably low level of impact strength. Moreover, the flow properties of such polymer compositions are very poor.
U.S. Pat. No. 3,111,501 discloses a polymer composition consisting of 68-85% of a copolymer of .alpha.-methylstyrene and acrylonitrile and 32-15% of a copolymer of 40-50 parts by weight of styrene and acrylonitrile grafted onto 60-50 parts by weight of polybutadiene. This polymer composition also exhibits very poor flow behaviour, caused by the high rubber content of the graft copolymer and the high percentage of copolymer which exhibits poor flow.
British Pat. No. 1,390,973 describes mixtures of ABS, SAN, and a copolymer of .alpha.-methylstyrene and acrylonitrile. According to this patent, problems relating to the flow behaviour of a mixture of ABS and a copolymer of .alpha.-methylstyrene and acrylonitrile can be overcome by adding SAN to the mixture. The improvement actually achieved, however, is fairly limited since it occurs at the expense of the heat resistance and impact strength of the composition.
U.S. Pat. No. 4,404,323 relates to a heat resistant polymer composition based on a graft copolymer obtained by polymerizing 50-80 parts by weight of a monomer mixture consisting of 20-40 % by weight of acrylonitrile, 50-80 % by weight of styrene and 0-30 % by weight of one or more other monomers in the presence of 20-50 parts by weight of rubber and a copolymer of .alpha.-methylstyrene and acrylonitrile obtained by polymerizing a mixture of 60-80 parts by weight of .alpha.-methylstyrene and 20-40 parts by weight of acrylonitrile, of which copolymer the spiral flow length is at least 65 and the flow index 45 at most, and the ratio between spiral flow length and the flow index is at least 3.0 and in which polymer compositions 35 to 80 % by weight of copolymer of .alpha.-methylstyrene and acrylonitrile is present.
The copending U.S. patent application Ser. No. 534,635, filed Sept. 23, 1983 relates to a polymer composition based on one or more graft copolymers of styrene and acrylonitrle on a rubber and a copolymer of .alpha.-methylstyrene and acrylonitrile. This polymer composition comprises one or more graft copolymers, obtained by polymerizing 20-80 parts by weight of a monomer mixture consisting of 20-40 % by weight of acrylonitrile, 50-80 % by weight of styrene and 0-30 % by weight of one are more other monomers in the presence of 20-80 parts by weight of rubber and one or more graft copolymers. obtained by polymerizing 75-90 parts by weight of a monomer mixture consisting of 20-40 % by weight of acrylonitrile. 50-80 % by weight of styrene, and 0-30 % by weight of one or more other monomers in the presence of 10-25 parts by weight of rubber and a copolymer of .alpha.-methylstyrene and acrylonitrile obtained by polymerizing 60-80 parts by weight of .alpha.-methylstyrene and 20-40 parts by weight of acrylonitrile, of which polymer composition the rubber content is between 10 and 30 % by weight, while the difference in rubber content of the graft copolymers is at least 5 % by weight.
For many applications that require ABS having improved flow properties, the reduced heat resistance that also results is still adequate. These are the so-called medium-heat ABS grades. However, the impact strength of the polymer composition of the above-mentioned German patent is so modest, particularly at low temperatures, that the range of applications is in practice very limited.
The object of the present invention is, therefore, to provide a polymer composition which exhibits better heat resistance than a graft copolymer of styrene and acrylonitrile on rubber, and which possesses a very high impact strength, particularly at low temperatures.